Acetic anhydride is a clear, colorless, liquid with a sharp, vinegar-like odor. Acetic anhydride is a commercially valuable chemical and can be found in a wide variety of industrial applications. Some primary applications of acetic anhydride include its use in the manufacture of cellulose acetate for films, plastic goods and coatings. Other applications include use in the manufacture of perfumes, explosives, synthetic fibers, sweeteners, polymers (e.g., polyoxytetramethylene glycol and polyacetal), weed killers, fungicides, various industrial chemicals, acetylsalicylic acid (aspirin), acetylcholine hydrochloride, acetophenacetin, sulfonamides, aceto-p-aminophenol, cortisone, acetanilide, theophylline, sulfa drugs, certain vitamins and hormones, and many other various pharmaceuticals and pharmaceutical intermediates not listed here. Acetic anhydride can also be used in the chemical treatment of papers and textiles and to produce acetyl ricinoleates, triacetin, acetyl tributyl citrate, and other plasticizers. Because acetic anhydride reacts with water, it is also sometimes used as a dehydrating agent in reaction mixtures where the removal of water is necessary.
Acetic anhydride may be prepared via the reaction of acetic acid and ketene. Ketene, a valuable intermediate for the synthesis of many organic compounds, may be prepared by the thermal decomposition (continuous cracking) of acetic acid in the presence of a catalyst, e.g., triethyl phosphate. This reaction may be conducted at temperatures at or above 600° C. and under pressures ranging from about 10 kPa to 50 kPa. Ammonia may be added to the mixture of hot cracked gas shortly after it leaves the reaction zone to neutralize the catalyst. Ketene is then isolated from the cracked gas and reacted with excess acetic acid to obtain crude acetic anhydride.
Typically, the ketene/acetic acid reaction is carried out in a system which includes an absorber and a scrubber. The majority of the ketene is reacted in the absorber stage. A crude acetic anhydride product stream exits the bottom of the absorber. Residual ketene, e.g., unreacted ketene, exits the absorber stage (via an overhead gas stream) and is typically directed to the scrubber to convert the unreacted ketene into additional acetic anhydride.
Various processes are known for purifying crude acetic anhydride. One conventional process requires the use of three columns, and, as such, necessitates a large capital expenditure. Attempts have been made to reduce the expenditure by reducing the number of columns. For example, U.S. Pat. No. 4,107,002 discloses the use of evaporators to evaporate the crude acetic anhydride into one portion comprising gaseous distillable components and another portion consisting substantially of non-gaseous non-distillable components. The gaseous distillable components are fed to a single distillation column to recover acetic anhydride at the lower part of the column. This separation scheme, however, requires at least one evaporator, which adds to both capital and operating expenditures.
Notwithstanding the above, there remains a need for a process for purifying acetic anhydride which eases the burden of capital expenditures associated with existing processes, and provides a high purity acetic anhydride.